vsat technology - Department of Computer Science and Engineering

VSAT
TECHNOLOGY
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VSAT Introduction
VSAT stands for Ve r y Small Aperture Terminal and refers to receive/transmit
terminals installed at dispersed sites connecting to a central hub via satellite using
small diameter antenna dishes (0.6 to 3.8 meter). Traditionally, the satellite is a radio
relay station that receives, amplifies and redirects analog and digital signals contained
within a carrier frequency. These signals contain data, voice, and video
communications. VSAT systems can be configured for bi-directional or receive-only
operation. In bi-directional operation, the dish both sends (uplinks) and receives
(downlinks) the information for use in LANs.
What is a Satellite?
Any object in the Solar system that revolves around another object that is either
static or in motion is a satellite to the latter. For e.g. Earth is a satellite to the Sun &
the Moon is a satellite to Earth.
How are satellites classified?
Broadly we can classify satellites into 2 types:
a) Natural Satellites: These are satellites that have been existing even before
existence of any living organism on earth e.g. Moon.
b) Man made satellites: These are satellites that have been placed into space by
human being to achieve a specific purpose. These satellites are sophisticated
electronic communications relay station orbiting around the equator moving in a
fixed orbit at the same speed & direction of the earth. These satellites like all living
things has a specific life time e.g. the INSAT series of satellites which have been
launched by India.
Why do we need man made satellites?
Man made satellite is used in a variety of areas like weather forecasting,
communication, navigation systems, television broadcasting etc.
What are the different kinds of man made satellite?
Based on the orbit in which a satellite is placed we can classify man made satellites
as:
a) LEO
LEO stands for Low Earth Orbit satellite. These satellites circle the earth at a distance
that varies from 100 to 300 miles. The orbit in which these satellites are placed is
called Polar Orbits. Leo's are also known as Polar Orbit satellites, a Polar Orbit
satellite travels from North-South direction. Since these satellites are very close to
Earth and to avoid getting pulled back into the gravitational pull of the earth, they
have to travel at speeds of 29,359 Kms/ hour and they circle the earth once in every
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90 minutes. Hence they have a rather short life span, as the amount of fuel it uses to
stay into orbit is very high. Polar Orbit satellites are mostly used for scanning the
Earth's surface. Some of the most
popular areas where Polar Obits satellites are used are Weather Satellite & Remote
Sensing satellites. Iridium is another example of LEO satellites.
LEO Satellites are deployed for Mobile / Satellite Telephony applications. Good
example is Iridium and ICO Global kind of networks.
b) MEO
MEO stands for Medium Earth Orbit satellite. These satellites circle the earth at a
distance that varies between 6,000 to 12,000 miles and would take approximately 5 to
12 hours to circle the earth once. MEO's are most popular in GPS services.
c) GEO
GEO stands for Geosyncronous Equatorial Orbit satellite. These satellites circle the
earth at a distance of 22,282 miles or 36,000 Kms. These satellites move at the pace
of the earth & will rotate at the same speed, as the earth. As the move at the same
speed of the Earth they appear to be stationary. A Satellite placed in the GEO Orbit
will take about 24 hrs to complete one rotation. These satellites rotate in an equatorial
orbit. Since GEO's move along with the earth's rotation they will cover the same area
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all their life. INSAT 2B and 2C are some examples of Geosynchronous satellites. It
takes about 3 satellite placed in the Geosyncronous Orbit to completely cover Earth.
What is an orbit?
The path, which a satellite takes to circle round its parent planet, is called Orbit.
What do these satellites consist of?
Man made satellites that orbit earth & the sun are highly sophisticated tools that
involve complex electronics however two main components to all the satellites are the
Payload and the Bus.
The payload is all the equipment; a satellite needs to do its job. This can include
antennas, cameras, radar, and electronics. The payload is different for every satellite.
For example, the payload for a weather satellite includes cameras to take pictures of
cloud formations, while the payload for a communications satellite includes large
antennas to transmit TV or signals to Earth.
The bus is the part of the satellite that carries the payload and all its equipment into
space. It holds all the satellite's parts together and provides electrical power,
computers, and propulsion to the spacecraft. The bus also contains equipment that
allows the satellite to communicate with Earth.
Who uses these satellites and for what applications?
Satellite services are used for a variety of applications. Satellites are used for weather
forecasting, TV Broadcast, GPS, Long Distance Telephony and Data
Communication.
How has satellite technology touched the life of a common man?
Television, one of mans greatest invention of all times uses satellite technology. TV
Broadcasters use satellite communication methods to ensure that you're favorite
program come to you on a flip of a single button on your remote. Have you ever
thought that without this satellite technology how would we ever have seen any
television channel at all? Why Satellite? Is another question that comes to mind.
The only way the TV Broadcaster can reach millions of people covering large spans
of territory is by using Satellite. This is not only economical but also cost effective
compared to any other medium.
Satellites again play a very important role in long distance telephony, rural telephony.
Farmers depend on weather reports for their crop. These are a few areas of life where
satellites have not only touched the life of a common man but also improved it for the
better.
What are the advantages of using satellite technology?
Some advantages of using satellite technology are:
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a) Very high reliability, all onboard systems are fully redundant.
b) Distance insensitive
c) High bandwidth capacity
d) No last mile issues
e) Speedy installation
f) Mobile, can be used for short term or emergency communications
g) Excellent for broadcast transmission
h) Bandwidth on demand
What are VSATs?
The term VSATs stand for Very Small Aperture Terminal, these are fixed satellite
terminals that are used to provide interactive or receive -only
communications.
Why are VSATs used?
VSATs are used for a wide variety of telecommunications applications such as
Corporate networks, Rural Telecom, Distance Learning, Telemedicine, Disaster
Recovery, Ship - Board communications (communication on large ships), etc.
Who uses VSATs?
VSATs have become increasingly popular, because they are a flexible
communication platform that can be installed quickly & cost effectively to provide
telecom solutions to consumers, governments & corporations.
VSATs have a wide range of users starting from large corporates with large value
chains having a wide geographical spread to smaller organizations which have office
in different locations, the defense establishments, stock exchanges, manufacturing &
FMCG companies are the typical users of VSAT's.
What are the advantages of using VSATs?
Some of the advantages of using VSATs are:
a) VSATs are highly reliable & boasts of uptimes as high as 99.5%
b) Since VSATs use a satellite to communicate geographical boundaries or terrain is
not a constraint.
c) A centrally managed network, which reduces a lot of logistics cost for the
customer.
d) In case of a failure the Mean Time to Repair is in the order of a few Hours.
e) No last Miles for the customer
f) Most important One Vendor Management.
What are the components that go into making a VSAT system?
Antenna
Power Amplifier
Up - Converter
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Down Converter
Modulator
Demodulator
CPE
Low Noise Amplifier
Antenna:
The antenna is responsible for transmitting, the amplified signal from the power
amplifier to the satellite and also receiving the signal from the satellite in conjunction
with the low noise amplifier. The Antenna is parabolic in size.
Antenna
Antennas are the passive equipment, which serve the purpose of directing a
transmission to a specific satellite as well as receiving the relevant transmission from
the same. The Antenna systems also provide the mechanical support for mounting the
RF units as well as the rest of the VSAT equipment configured for outdoor mounting.
Antennae are specified for the frequency band of operation, directional gain, aperture
efficiency levels and the accuracy of orientation in the specific frequency.
The antenna sizing for VSATs is one key aspect of Link design. The sizing depends
on Frequency of operation: Antenna size varies in inverse proportion to the frequency
of operation for a given set of specifications like directional gain. A C/Ext C band
antenna with the same features shall be larger than a Ku band antenna.
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Feed horn is a part of Antenna. It plays a vital role in a VSAT system. It receives
signal from the Satellite reflected to Antenna and fed to the Indoor unit. Also, the
signal which to be send to the Satellite is also thrown through this feed horn so that
the signal reflecting to the reflector should reach Satellite.
Working of feed horn and reflector
Power Amplifier:
The Power Amplifier is used for amplifying the Up converter RF signal before being
fed into the Antenna system. The Amplifier can be either Mounted on the Antenna
system or could be placed in the Indoor Rack. The amplification is required to send
the up stream signals to the Satellite.
Low Noise Amplifier:
The signal that travels from the satellite would have become weak due to various
atmospheric issues, the signal strength is reduced to a few watts hence the signal need
to pass through an equipment that will increase the signal strength from a few watts to
several Kilowatts.
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The low noise amplifier is responsible for amplifying very low power satellite signals
received at the antenna to a higher signal strength before it is fed into the down converter.
Down-Converter:
A down - converter amplifies and converts the frequency (RF to IF), which is
received from the low noise amplifier. This is then passed on to the demodulator.
Up-Converter
An up-converter amplifies & converts the frequency (IF to RF), that is received from
the modulator. This is then passed on to the power amplifier for further amplification
and transmission.
Demodulator & Modulators:
Demodulator is responsible for converting the IF signals into digital format. This is
understood by the networking components like Routers, Switches, Telephone
systems, etc. and the same is then fed into the computer.
Modulators on the contrary are responsible for converting the digital data into IF
signals.
What is a VSAT hub?
A VSAT hub is a huge earth station that is responsible for controlling & monitoring
all
the activities of the geographical spread of VSATs.
In some cases all the remote VSATs communicate to one central site, this Central
Site is connected to the hub, as the Hub is the switching element.
Multiplexing Techniques
A satellite link can relay signals from a single earth station. These signals must be
separated to avoid interfering with each other. This separation is called multiplexing.
The most common forms of multiplexing are:
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a) FDM - Frequency Division Multiplexing
A group of signals pass through the same channel but on different frequencies.
b) TDM -Time Division Multiplexing
A group of signals take turns in different time intervals to use a channel.
Theoretically either multiplexing technique can be used with analog or digital
modulation. But, TDM is more easier to implement when the content is digital. This
is because digital signals are precisely timed and consists of groups of short pulses
with relatively long intervals between them. FDM is more convenient with analog.
Multiple access is "the ability of a large numbers of remote stations to simultaneously
interconnects their respective voice, data, Teletype, facsimile and television links
through a satellite".
The multiple-access is fundamental to satellite communication because it is the means
by which the wide geographical coverage capability and broadcast nature of the
satellite channel are exploited. It affects all the elements of the system, determines the
system capacity and flexibility, and has a strong influence on costs. It involves how to
permit a changing group of remote stations to share a satellite in a way that optimizes
(1) satellite capacity
(2) spectrum utilization
(3) satellite power
(4) interconnectivity
(5) flexibility
(6) Adaptability to different traffic mixes
(7) Cost
(8) user acceptability
Usually all the elements in this list cannot be optimized and some may have to be
traded off against others.
Classically there are three main multiple access techniques, they are:
a) FDMA (Frequency Division Multiple Access) - All the users share the satellite at
the same time. But, each transmits in its own unique frequency band. This is
most commonly employed with analog modulation, where signals are present all
the time.
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b) TDMA (Time Division Multiple Access) - All the users transmit in turn in their
own
unique time slots. While transmitting, each occupant has exclusive use of one or
more transponders. The intermittent nature of TDMA transmission makes it
particularly attractive for digital modulation.
c) CDMA (Code Division Multiple Access) -Many earth stations simultaneously
transmit orthogonal coded spread-spectrum signals that occupy the same frequency
band. Decoding ("de-spreading") systems receive the combined transmissions from
many stations and recover one of them.
In all the three classical multiple access schemes some resource is shared. If the
proportion allocated to each earth station is fixed in advance, the system is called
fixed access (FA) or Pre-assigned Access (PA). If the resource is allocated as needed
in response to changing traffic conditions, the multiple access arrangement is termed
Demand Access (DA).
Frequency Division Multiple Access (FDMA) - FDM/FM/FDMA
Frequency division multiple access with FM frequency division multiplexing is
abbreviated as FDM/FM/FDMA. In it a remote station is permanently assigned a
carrier frequency.
The station frequency modulates all its outgoing traffic, whatever the destination, on
that carrier. An originating station's traffic capacity is limited by its allocated
bandwidth and the carrier to noise ratio (denoted as C/N) that it can achieve on the
downlink. The carrier frequencies and bandwidth assigned to all the remote stations
constitutes a satellite frequency plan.
Every station that operates in an FDM/FM/FDMA network must be able to receive
atleast one carrier from all the stations in the network. Thus most FDM/FM/FDMA
stations have a large number of separate IF receivers & de- multiplexers. Satellite
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FDM/FM/FDMA were patterned after the terrestrial analog telephone microwave and
cable transmission systems used in the early days of the Intelsat system.
Some common example of the FDMA system is Our PAMA & DAMA services.
Time Division Multiple Access:
In time division multiple access (TDMA) a number or earth stations take turns
transmitting bursts through a common transponder. Since all practical TDMA
systems are digital, TDMA has all the advantages over FDM/FM/FDMA that digital
transmission usually has over analog.
TDMA is easy to reconfigure for changing traffic demands, resists noise and
interference, and mixes voice and data traffic. But, one advantage of TDMA systems
is that it permits a transponder's TWT (Traveling Wave Tube) to operate at or near
saturation and thus it maximizes the downlink (C/N).
Since only one carrier is in the TWT at a time, there are no inter-modulation products
to worry about and no back off is necessary. Many of the concepts for time division
multiplexing (TDM) apply without change to TDMA.
In TDM digital data streams from many sources are transmitted sequentially in
assigned time slots; the slots are organized into frames that also contain
synchronization information. A receiving station must first recover the transmitter
carrier frequency, then recover the transmitting station clock pulses, and then identify
the start of each frame so that it can recover each transmitted channel and route it on
to its destination. The principal difference is that in TDM everything comes from the
same transmitter. The clock and the carrier frequencies do not change.
While, in TDMA each frame contains a number of independent transmission. Each
TDMA station has to know when to transmit, and it must be able to recover the
carrier & clock for each receive burst in time to sort out all wanted baseband
channels.
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TDMA Frame Structure & Design :In TDMA transmissions a group of earth stations, each a different distance from a
satellite, must transmit individual bursts of RF energy in such a way that the bursts
arrive at the satellite in a prescribed order. The stations have to adjust their
transmissions to compensate for the variations in satellite range, and they must be
able to enter or leave the network without disrupting its operations.
These goals are accomplished by organizing TDMA transmissions into frames
containing reference bursts that establish absolute time for the network. Each station
transmits once per frame so that its burst begins to leave the satellite a specified time
interval before or after the start of a reference burst.
Each frame contains one (or two for redundancy) reference burst and a series of
traffic bursts. Each traffic burst contains a preamble, which provides synchronization
(sync) and signaling information and identifies the transmitting station, followed by a
group of traffic bits. The traffic bits are the revenue-producing portion of the frame,
and the reference bursts and the preamble constitutes system overhead. The smaller
the overhead, the more efficient a working TDMA system is, but the difficulty it may
have in acquiring and maintaining sync.
Code Division Multiple Access
Code Division Multiple Access (CDMA) is a scheme in which, a number of users
occupy all of a transponder bandwidth all of the time. Their signals are encoded so
that information from an individual transmitter can be detected and recovered only by
a properly synchronized receiving station that knows the code being used. This
provides a decentralized satellite network, as only the pairs of stations that are
communicating need to coordinate their transmissions.
Subject to transponder limitations and the practical constraints of the codes in use,
stations having traffic can access a transponder on demand without coordinating their
frequency (as in FDMA) or their time slot (as in TDMA) with any central authority.
Each receiving station has its own code called its "address", and a transmitting station
simply modulates its transmission with the address of the intended receiver whenever
it wishes to send a message to that receiver.
CDMA is most suited for a military tactical communication environment where many
small groups of mobile stations communicate briefly at irregular intervals than to a
commercial environment where large volumes of traffic pass continuously between a
small number of fixed locations.
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Mesh and Star networks
VSAT's are connected by radio frequency links via a satellite. Those links are radio
frequency links with a so-called `unlink' from the station to the satellite and a socalled `downlink' from the satellite to the station. The overall link from station to
station, sometimes called a Hop, consists of an uplink and downlink. A radio
frequency link is a modulated carrier conveying information. Basically the satellite
receives the unlinked carriers from the transmitting earth stations within he field of
view of its receiving antenna, amplifies those carriers. Translates their frequency to a
lower band in order to avoid possible output / input interference, and transmits the
amplified carriers to the stations located within the field of view of its transmitting
antenna.
Present VSAT networks use Geo-stationary satellites and as a result all the VSAT's
are visible from the satellite all the time, carriers can be relayed by the satellite from
any e VSAT to any other VSAT in the network. These are nothing but Mesh
networks. However in mesh networks one must take into account the following
limitations:
· Typically 200 dB carrier power attenuation on the uplink & the downlink as a result
of the distance to and from a GEO-stationary satellite
· Limited satellite radio frequency power, typically few tens of watts
· Small size of the VSAT, which limits its transmitting power and its receiving
sensitivity
As a result of the above limitations, it may well be that the demodulated signals at the
receiving VSAT do not match the quality requested by the user terminals. Therefore
direct links form VSAT to VSAT may not be acceptable.
The solution then is to install in the network a station larger than a VSAT, called the
Hub. The Hub station has a larger antenna size than those of a VSAT, say 4 meters to
11 meters. This results in higher gain than that of a typical VSAT antenna, and it is
also equipped with a more powerful transmitter. As a result of its improved
capability, the hub station is able to receive adequately all the carriers transmitted by
the VSATs, and to convey the desired information to all the VSATs by means of its
own transmitted carriers. These are nothing but Star networks.
The links from the Hub the VSAT are named `outbound links'. The ones from the
VSAT to the Hub are named `inbound links'. Both inbound and outbound links
consist of two links, uplink and downlink to & from the satellite.
There are two alternatives to star shaped VSAT networks:
· One-way networks: where the hub transmits carriers to receive only VSATs. This
configuration supports broadcast services from a central site where the hub is located
to remote sites where the receive-only VSATs are installed.
· Two-way networks: where VSATs can transmit & receive. Such networks support
interactive traffic.
One Way connectivity happens where there is a broadcast from the central location to
all the remotes.
The two-way connectivity between VSAT's can be achieved in two ways:
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Either direct links from VSAT to VSAT via the central Hub, hence making it a
double hop scenario, with a first hop from VSAT to hub and then a second hop using
the hub as a relay to destination VSAT. The second way is by single hop links via
satellite in a star shaped network
In conclusion, star shaped networks is imposed by power requirements resulting from
the reduced size and hence the low costs of the VSAT earth station in conjunction
with power limitation of satellites. Meshed networks are considered whenever such
limitations do not hold, or are unacceptable. Meshed networks have the advantage of
reduced propagation delay (single hop delay is 0.25 sec's instead of 0.5 sec's for
double hop) which is especially of interest for telephony services.
What are the different access methods used in VSAT communication?
Various Access methods used in VSATs to communicate with each other are:
1. SCPC Single Channel Per Carrier ( In simple terms this is nothing but lease lines in the sky).
SCPC Channels can be either PAMA or DAMA.
2. TDMA
PAMA: Pre Assigned Multiple Access
PAMA is an access scheme where in when 2 VSATs want to communicate with Each
other a bandwidth is assigned to them exclusively. This assigned bandwidth will Be
available the VSAT's on a permanently basis. This link can either be a symmetric and
asymmetric link. It is nothing but a point to point connectivity.
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Point to Point connectivity, Leased Line in the sky
The PAMA service interconnects high data traffic sites within an organization. It is a
cost-effective alternative to terrestrial leased lines, providing high reliability links to
support mission critical applications. DAMA: Demand Assigned Multiple Access The
DAMA scheme is very similar to a telephone connection. Whenever, there is a need
to talk to someone, you dial a number. The call lands at the telephone exchange, and
the telephone exchange connects you to the dialed number.
The role of the telephone exchange is to connect you to the desired number. This is
exactly how a DAMA network operates. The HUB plays the role of a telephone
exchange, between any two VSAT's. The DAMA service addresses point to point
voice, fax, and data communication requirements of remote sites. It provides a cost
effective and reliable solution to business having a high internal voice/ fax
communication requirements. Additionally it enables organizations with operations in
remote areas, to establish a reliable communications network.
Typical DAMA / PAMA Network
TDM/TDMA: Time Division Multiplexed/Time Division Multiple Access The
TDMA network operates in a Star topology. All the remote VSATs communicate to
the central hub station, on a Time Division Multiple Access Modes. At the hub the
signal is re -transmitted to the destination VSAT using TDM technology after
amplification. The Access mechanism of TDMA operates on a technology called
Slotted Aloha. All the remote VSAT's contend for a time divisional slot to transmit
their packets to the Hub. The channel used by the remotes to communicate to the Hub
is called the Return Link. Each of these return channels operates at a maximum of
128 Kbps.
The Hub communicates to all the destination remotes using the TDM technology.
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The communication channel from the Hub to the remote is also called the Outbound
or Outlink. The outbound or outlink from operates at 256 Kbps.
In TDM/TDMA the implementation topology is a Hub & Spoke architecture here all
the remote sites communicate to the central site via the HUB. The Hub is connected
to the central site on an SCPC connection.
Typical applications on TDMA
· Interactive Data
Enterprise Resource Planning solutions like SAP, BPCS, BAAN, JD Edwards, to
name a
few have been implemented on TDMA VSAT network. These solutions require
interactive data communication between remote sites and the central host site. The
network is also suited to carry intra-office e-mail traffic from cc: Mail, MSMail
amongst others.
The VSATs support multiple protocols enabling them to interface with existing
customer networks.
· Data Broadcast
Continuous data broadcast to a large number of locations as in a stock exchange
application or occasional file transfer from a central location to multiple remote
locations is supported on the TDMA VSAT network. This is supported using the
Validate system, which is part of the TDMA VSAT system. The Validate system
provides both confirmed data broadcast, as required in file transfer applications and
also unconfirmed data broadcast to meet continuous data feed transmission.
· Occasional Voice on TDMA networks
The TDMA systems also offer voice communication support. This is suitable to
interconnect remote locations. Various encoding rates are offered ranging from 4.8
to 16 Kbps. This gives the customer a choice to choose the appropriate voice quality
as per the requirement.
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What are the different bands available on a satellite?
Any Satellite has different frequency bands available on it the table below shows
what all bands are available with its operation frequencies in which the satellite up
links & down links.
SL.NO
BAND
C –BAND
UP LINK RANGE
(GHZ)
5.925-6.425
DOWNLINK
RANGE (GHZ)
3.700-4.200
1
2
EX C BAND
6.725-7.025
4.500-4.800
3
KU BAND
14.00-14.50
10.95-11.70
4
KA BAND
30.00
20.00
What are the advantages & disadvantages of each band?
BAND
ADVANTAGE
DISADVANTAGE
C-BAND
Broad footprint
Little rain fade
Interference
Large Antenna and Amplifier
EX-C -BAND
Broad footprint
Little rain fade
Less Interference
Weak signals
Large Antenna size
Large Amplifier
KU -BAND
KA-BAND
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Focused footprint
Less terrestrial Interference
Smaller Antenna
Smaller Amplifiers
Focused footprint
Less terrestrial Interference
Smaller Antenna
Smaller Amplifiers
Interference due to rain
Interference due to rain
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What is a footprint?
The area that is covered by the beam of a satellite is called a footprint. For e.g. in the
figure above the marked area is a footprint of a satellite.
Different kinds of footprints are:
· Global Beam: coverage of entire surface of the earth that is visible by the satellite.
· Hemisphere Beam: coverage only of the hemisphere region.
· Spot Beam: coverage only on a particular region, e.g. Coverage only of the Indian
sub-continent
What is Rain Fade?
Rain Fade is an interruption of Wireless communication signals. As a result of rain or
snow droplets whose separation approximates the signal wavelengths. This
phenomenon can effect satellite connectivity and all satellite based communication.
Rain fade usually does not last long. Once a heavy shower or squall has passed,
normal communication returns. However, during tropical storms or severe winter
storms at northern latitudes, fadeouts can persist for hours at a time. The phenomenon
occurs with all types of satellite systems.
Coding and Modulation
Modulation
Modulation is a technique where in baseband data is superimposed on a carrier for
transmission. There are different modulation techniques that have evolved over the
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years. However in digital data transmission the most primitive modulation technique
is called PSK (Phase Shift Keying). The reason why its called phase shift keying, is
because in digital data, the data are in 0's or 1's. This data is represented by the phase
relationship of the RF carrier to itself or to a reference.
For example the phase offset of the carrier in one direction may represent one type of
data, then, a phase offset in the opposite direction may represent another type of data.
There are different types of modulation techniques that have evolved over the years
for digital data transmission. They are:
a) BPSK: Binary Phase Shift Keying
In the BPSK modulation technique the zeros and ones are represented by two phases
of the RF carrier signal, which differ by 180 degrees.
b) QPSK: Quadrature Phase Shift Keying
In QPSK modulation, zeros & ones are represented by four phases of the RF carrier,
each differing by 90 degrees from the next.
c) 8 PSK: 8 Phase Shift Keying
In 8 PSK modulation, zeros & ones are represented by 8 phases of the RF carrier,
each differing by 45 degrees from the next.
d) 16 PSK: 16 Phase Shift Keying
In 16 PSK modulation, zeros & ones are represented by 16 phases of the RF carrier,
differing by 22.5 degrees from the next.
e) M-PSK: Multi Phase Shift Keying
In M-PSK modulation, zeros & ones are represented by multiple phases of the RF
carrier. The difference would vary in accordance to the output required.
As the difference in the phase shifts increase, the probability of increase in the error
becomes higher. Due to this reason the most widely used modulation techniques are
BPSK & QPSK modulation.
In QPSK modulation, two information bits are encoded at one time. This means that
when transmitting the data in QPSK, the phase of the RF carrier must change at only
half the rate.
Both BPSK & QPSK are extremely efficient modulation techniques. With careful
filtering techniques, bit error rate (BER) performance of 1 to 2 dB of the theoretical
limit may be achieved. To achieve this low error rate, one approach is to filter the
baseband or digital data before modulation with a Nyquist filter. Such a filter not only
allows optimum performance to be achieved, but also constraints the PSK signal to
the minimum possible bandwidth.
Advantages of each approach
An analysis of PSK modulation shows that the theoretical performance of BPSK and
QPSK modulation is identical in a channel dominated by Gaussian noise, such as a
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satellite channel. This allows the choice to be made between BPSK and QPSK based
on other considerations.
With QPSK, the transmitted spectrum occupies only half the bandwidth of BPSK, and
would therefore be a good choice in an environment where bandwidth efficiency is
required.
The prime advantage of BPSK is that it is much more tolerant to phase noise than
QPSK. If the system is designed from the beginning with BPSK in mind, then lower
cost microwave equipment can be used in the up and down conversion process,
without compromising performance. Likewise, in a burst mode system, BPSK has a
second advantage over QPSK as the burst demodulator takes shorter acquisition time.
This allows the frame overhead to be kept to a minimum leading to increased
efficiency while utilizing a lower cost transponder.
Hence when designing the overall system, the designer tries to make optimum use of
the satellite characteristics. There are three factors that have to be borne in mind when
designing a particular system and choosing the modulation scheme:
· Satellite limitations
· The total power of all the desired carriers must not exceed a certain power level.
· The total bandwidth of all the desired carriers must not exceed the bandwidth of the
transponder.
· Hardware costs
· System Goals
Forward Error Correction (FEC)
In forward error correction a few coding bits are added to the actual information data
stream. The added bits have an in built mechanism to identify & rectify errors at the
receiving end. This is done to achieve good bit error rates and low carrier to noise
ratio.
There are different techniques used in FEC starting from ½, ¾, 5/6, 7/8, etc.
Here the numerator denotes the actual information bit & the denominator denotes
information bit + coding bit. When ½ FEC is used it means that to every 1 bit of
actual information 1 coding bit is used. Similarly when 5/6 FEC is used 1 coding bit
is added to every 5 information Bits.
Bit and Symbol Error Rates
Bit Error Rate (BER) is also called the Bit Error Probability (PB). Mathematically
this is the probability that a bit sent over the link would be received incorrectly (i.e.
that a 1 will be read as a 0 or vice versa) or, alternatively, the fraction of a large
number o f transmitted bits will be received incorrectly. Like a probability, it is
usually stated
as a single number -
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for example 1 x 10-4 or .001. The BER plays the same role as an indicator of quality in
a digital communication system that the signal- to-noise ratio plays in an analog link.
Physically a bit error occurs because a symbol error has occurred. At some point in
the link noise has corrupted the transmitted symbol so that the decision circuitry at the
receiver cannot identify it correctly. For example, the carrier phase may have been
transmitted as +90 degrees but additive noise may have changed the received carrier
phase to -90 degrees.
VSATs in the Indian context
What is ISRO's role been in the Indian satellite industry as well as in the VSAT
industry?
ISRO stands for Indian Space Research Organization, setup in June 1972 under the
Department o f Space program. The primary objective of ISRO is to develop
satellites, launch vehicles, rockets and associated ground systems.
Some of the achievements of ISRO over the years has been launch of satellites
Aryabhata, Bhaskara, Rohini, INSAT series, IRS, etc. ISRO has also been the key
organization behind the development of satellite launch vehicles like the PSLV (Polar
Satellite Launch Vehicle), GSLV (Geosyncronous Satellite Launch Vehicle) etc. This
in a nutshell has been ISRO's role in the Indian Satellite industry.
What are the regulatory bodies that govern VSAT Service providers?
The various bodies that govern the VSAT Service providers and lays down rules &
norms to be followed by them are:
a) TRAI: Telecom Regulatory Authority of India
TRAI is an autonomous governing body that lays down guidelines &
recommendations to DOT on policy making. However weather these inputs are
followed by DOT is at the discretion of DOT. However TRAI is not a body that is
directly involved in governing any service providers.
b) DOT: Department Of Telecommunication
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DOT has a role in policy making, licensing and co ordination of matters related to
telegraphs, telephones, wireless, data, facsimile and telematic services and other
forms of communication. In addition DOT is responsible for frequency management
in the field of radio communication in close coordination with international bodies. It
also enforces wireless regulatory measures for wireless transmissions by wireless
users in the country.
c) WPC: Wireless Planning & Coordination wing
WPC is a part of DOT created in 1952 under the Ministry of Communications, is the
national radio regulatory authority responsible for frequency spectrum management,
including licensing and caters to the need of all wireless users in the country
government or private, security or non-security. It is also the national nodal agency
for all matters related to ITU (International Telecommunications Union) and the Asia
Pacific Telecomm unity (APT) and is responsible for treaty obligations on behalf of
the Government of India. It also exercises the statutory functions of the Central
Government and issues licenses to establish, maintain and operate wireless stations as
well as possess, develop and deal in wireless equipment in the country.
d) SACFA: Standing Advisory Committee on Frequency Allocation
The Standing Advisory Committee on Frequency Allocations (SACFA) is a high
level committee chaired by secretary (DOT)/Chairman, Telecom Commission. Heads
of major wireless users/administrative ministries of the govt. of India, Member
(Technology), Telecom Commission, and Wireless Adviser to the govt. of India, Joint
Secretary, DOT are its members. WPC wing of the ministry of communications
provides secretariat help to the committee. Joint Wireless Adviser, WPC wing is the
member-secretary of the committee.
The main functions of the committee are to make recommendations on:
· Major frequency allocation issues,
· Formulation of National Frequency Allocation Plan,
· Making recommendations on various issues related to International
Telecommunications Union (ITU)
· Asia Pacific Telecomm unity (APT),
· To sort out the problems referred to the committee by various wireless users, siting
clearance of all wireless installations in the country, etc. SACFA clearances are
issued after getting 'no objection' from various SACFA members who have to carry
out detailed technical evaluation including field surveys, etc. at times they have to
obtain evaluations from their field units. The technical evaluation is done primarily
for:
a) Aviation hazards.
b) Obstruction to line of site of existing/planned networks
c) Interference (Electro Magnetic Interference (EMI)/Electro Magnetic Compatibility
(EMC)) to existing and proposed networks.
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TYPICAL VSAT INSTALLATION
Site Survey
Before installation of any VSAT we need to know some parameters which are very
critical. To get these parameter we have to do survey of the site where VSAT to be
installed. The general parameters are :1. Look angle of the Antenna - VSATs will send and receive RF signals from
respective satellite. Hence, we have to find out the coordinates of the site. The
coordinates indicates latitude and longitude of the site. To get this Lat and
Long information site survey engineers use GPS equipment. GPS is Global
Positioning System which gather information from satellites the lat and long
information. Calculating the coordinates of the site and coordinates of the
satellite we can find out the look angle of the VSAT. This look angle consists
elevation ( vertical ) and azimuth ( horizontal ) angles.
2. Line of sight ( LOS ) – There must not be any obstruction from VSAT to
respective satellite. Hence , clear LOS is essential.
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3. Base – VSAT Antenna system placed with the support of ballast. Ballasts are
nothing but stones or bricks which are placed to keep the Antenna stable
against wind, thrust etc. Physically stability is very much required for a stable
network connectivity. Hence, the base ( roof top or ground ) must be flat and
strong to withstand the load of the VSAT Antenna. If the base is slanting then
there is a risk slippage or if the base is not strong enough then damage of the
base may happen. At the time of site survey all these information is very much
required. A 3.8 meter PAMA outdoor unit may weight 3000 kg and under
wind pressure at the time of thunder storm may rise subsequently.
4. Length of the IF cable from Outdoor Unit to Indoor Unit – The shortest length
is better. We have to found the route of the cable so that the cable should be
safe and should travel shortest path from ODU to IDU
5. Electrical Interference – The RF signal is prone to interfere with other signals.
High voltage electrical system or cable should not be around VSAT system.
The survey must find out best place to keep ODU and IDU free from electrical
interference.
6. Working space – some time it has been found that the best place to keep a
VSAT ODU is such that there is not space to work for servicing or
installation. Safety of the engineers may be an issue for such case. We must
keep this into mind at the time of survey.
7. Electrical Earthing – All communication equipment are prone to damage
against floating current. If the Earthing is not proper then this floating current
may cause damage to the electronic parts of the VSAT system. In general
Earth to Neutral voltage must be below 2 volts, so that negligible current will
flow. Besides, Neutral should be strong, so that though Earthing may be
perfect but due to weak neutral floating current may generate and cause of
damage to the equipments.
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